1. Field of the Invention
This invention relates to auxins, growth hormones for plants and, more particularly, to the new use of indole-3-succinic acid and its derivatives as an auxin.
2. Art Relating to the Invention
Auxins are well-known plant growth or development hormones, that were first extensively studied in the mid 1930xe2x80x2s. Auxins are involved in a variety of plant activities although their ability to promote cell elongation is perhaps best known. The most widely occurring, natural auxin in indole-3-acetic acid (IAA). It occurs in both free and conjugated states in plants and seeds. Early on, the use of IAA was shown to be advantageous in stimulating root formation in plant cuttings. Subsequently, synthetic materials such as indole-3-butyric acid (IBA) and naphthleneacetic acid (NAA) were found to be even more useful at least in part, due to their greater stability. Most recently it has been found that IBA also occurs naturally in some plants albeit at very low levels. Today IBA and NAA are widely used as synthetic rooting hormones. They are most often applied to the base of plant (stem and leaf) cuttings, and to transplantings since it is known that auxins are required for initiation of adventitious roots on stems, and to stimulate root growth in general. Rooting hormones are widely used for plant propagation because they hasten root initiation, improve rooting percentages, produce more uniform rooting, and increase the number and quality of roots.
It has now been discovered that indole succinic acid (indole-3-succinic acid) and its derivatives can be used as an auxin.
Hereinafter, the term ISA will be used to refer to indole-3-succinic acid and it derivatives while the individual names as recited herein will be used when referring to an individual compound, such as indole-3-succinic acid.
It has also been discovered that ISA is more effective in promoting growth of some seedlings than either the natural auxin IAA or the synthetic auxins IBA and NAA.
It has furthermore been discovered that both enantiomers alone of ISA and the racemate (racemic mixture) of ISA act as auxins and that either enantiomer alone of ISA and/or the racemate of ISA have a greater effect on root growth than either IAA or IBA. It has also been discovered that enantiomers ISA have different effects on the root growth of different plants.
Furthermore, it is surprising and unexpected that the enantiomers and the racemate have different effects on plant growth than each other.
Broadly, the present invention is directed to the new use of ISA as an auxin. The present invention is a method for promoting growth in plants comprising treating a plant with an effective amount of ISA to promote growth in the plant.
ISA which are capable of this new use, can be represented as follows: 
wherein:
Y is a hydrogen (H), a hydroxyl group (OH), a halogen, a nitro group (NO2), a sulfinate group (SO3), an alkyl group or an aryl group, and
X is a carboxylic acid group (COOH), a carboxylic acid ester group (COOR1), an acetyl group (CH2COOH) or an aryl group.
*The asterisk denotes a stereogenic center which means the compound can exist in two different enantiomeric forms.
Suitable halogens include fluorine (F), chlorine (Cl) and bromine (Br).
Suitable ester groups (R1) include alkyl groups.
Suitable alkyl groups for both Y and R1 include C1 to C6 alkyl groups, straight chained or branched, i.e. methyl, ethyl, propyl, butyl, pentyl and hexyl.
Suitable aryl groups include both one and two aromatic rings, e.g. phenyl and naphthyl.
Specific ISA""s that can be employed in the present invention include: 
As noted above, ISA is much more effective in promoting growth of some seedlings than either the natural auxin, IAA, or the widely used synthetic hormones IBA and NAA. However, unlike the other rooting hormones, ISA contains one stereogenic center, and can exist in two enantiomeric forms. Most chiral biologically active compounds are known to be stereoselective and it is known that enantiomers can have different biological actions and potencies. However, there have been few reports on enantioselective growth-promoting auxins. Indole-3-succinic acid has been synthesized only as the racemate, it has never been resolved into individual enantiomers, until now.
It has been discovered that the auxin activity of ISA is stereoselective, and both ISA enantiomers individually and the racemate act as auxins and can have the same or greater effect on root growth than either the natural auxin, IAA, or the most widely used synthetic analogue, IBA.
Treating the plant with ISA to promote growth in the plant is accomplished in a conventional manner using conventional equipment. Suitably, the plants can be grown hydrophonically, or the plant can be dipped into a solution of the auxin or still further the soil in which the plant grows can be treated with the auxin. The seed can also be coated with the auxin in a conventional manner with the auxin, however, this does not appear to work as well as the other three methods.
Treating the soil-is accomplished in a conventional manner using conventional equipment. It can be plowed into the ground, potting soil can be used which contain ISA or the hole into which the plant is placed can be sprayed with ISA.
The amount of ISA administered to the plant is an effective amount to promote growth in the plant. Suitably, the amount administered to the plant is a conventional amount, conventional in the sense of comparable to the amount of known auxins that are administered to plants to promote growth, however, lesser amounts can be used of ISA than conventional auxins to obtain comparable results in certain instances.
Suitably, ISA is administered as an aqueous solution either alone or with other additives. The optimum concentration range for ISA in solution is about 10xe2x88x925 to about 10xe2x88x929 M when the plant is grown hydroponically.
When the plant is dipped into an aqueous solution, the concentration of ISA in the solution is suitably about 0.001 to about 0.1% (weight percent). When the soil is treated, either by plowing it into the ground, adding it to the potting soil, or just spraying it into the ground into which the plant will be grown, the concentration of the ISA in the aqueous solution is suitably about one part per thousand to about one part per trillion (about 103 ppm to about 10xe2x88x926 ppm).
Other additives which can be included in the aqueous solution include nutrients such as nitrogen or phosphors, fungicides, herbicides and insecticides.
As noted, ISA can be administered as either enantiomer alone or as a racemate. ISA is suitably administered as a salt or any conventional form. ISA can be administered with acceptable carriers as is conventional with auxins.
ISA as well as the salts are made in a conventional manner using conventional equipment. The separation of the individual enantiomers from the racemate is likewise accomplished in a conventional manner using conventional equipment.
The term racemate or racemic mixture is an equal mix of both enantiomers. However, a mix in any amount or proportion of the two enantiomers can be used as well. Likewise, the ISA need not be pure enantiomers or pure racemate.
It has been found that racemic ISA is easily synthesized and can be resolved both chromatographically and by crystallization as the diastereomeric cinchonidine salt. The absolute configuration of its enantiomers can be determined by x-ray diffraction. Both enantiomers of ISA and its racemate have significantly greater xe2x80x9croot growth promoting activityxe2x80x9d than the popular IBA and the naturally occurring auxin IAA on the plants-tested. It has also been found that the (R) and (S)-enantiomers of ISA can have different activities with respect to plant growth.
Since different plants do not respond in the same way to all auxins, in some plants it will be beneficial to use racemic ISA. This is because a mixture of these two stereoselective growth promoters can elicit a broader range of responses than other conventional synthetic auxins that currently are-in use.